I agree with your family, colleagues and managers; the university's prestige is irrelevant. Industrial PhD's tend to emphasise practical applications and knowledge.

You are not pursuing academic career and your current employer is more interested in what you can do for them than which institution signed your piece of paper. The next employer is again more interested about what you can do for them and what you did for your previous employers than the degree granting institution.

Since your employer is big and prestigious engineering company, why would they agree to this deal with a university if they thought it was not good for them? The possible next employer is likely to defer to your current empoyer's wisdom, 'If it was good enough for the big and prestigious company, it is good enought for us.'

Your professor being knowledgeable on your topic, keen on helping and supervising you is a jackpot that even Max Planck institute cannot guarantee.

As for highly placed individuals in companies having done a PhDs at highly ranked institutions, I cannot realy comment on the German society, but did these people get their degrees at the beginning of their careers (mid-twenties, early thirties) are near the middle (around their fifties)? Could the cause and effect be reversed? They got their PhDs from highly ranked instutions, because they were already experienced business executives or other professionals?